#include<iostream>
#include<math.h>
#include<string.h>
#include<vector>
#include<algorithm>
#include<iomanip>
#include<deque>
#include<map>
#include<stdio.h>
using namespace std;

int main(){
	double px,py;
	cin >> px >> py;
	double qx,qy;
	cin >> qx >> qy;
	
	double dx=(px-qx);
	if(dx<0)dx*=-1;
	double dy=(py-qy);
	if(dy<0)dy*=-1;
	
	double dist=(dx+dy)/2;
	cout << dist << endl;

	return 0;
}